1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to interim telecommunications number portability.
2. Description of the Related Art
2.1 Terminology
Number Portability (NP) provides a subscriber with the ability to move from one End Office (EO) to another EO while keeping the subscriber's original Directory Number (DN). There are three types of Number Portability: Service Provider Portability, Geographic Portability, and Service Portability. Service Provider Portability enables a subscriber to change Local Service Providers while maintaining the same Directory Number and while ensuring no performance degradation between service providers or carriers. A Local Service Provider (LSP) is a provider of local access functionality to an end customer. LSPs are synonymous with Local Exchange Carriers (LECs), Alternate Local Exchange Carriers, Competitive Access Providers, and other terms that may be used to refer to providers of local access. Geographic Portability enables a subscriber to physically move within some reasonably defined region, for example, moving within the same Local Access and Transport Area (LATA), and to keep the same directory number. In such a situation, the subscriber may or may not change service providers. Service Portability enables changing service type, for example, POTS to ISDN, while maintaining the same directory number.
Presently, a call is routed based on the NPA-NXX number of the called party number. An NPA (Numbering Plan Area) number is the initial group of 3 digits of a North American Numbering Plan (NANP) number. The format of an NPA number is NXX, where N is any digit from 2 to 9 and X is any digit from 0 to 9. Currently, the NPA-NXX number identifies the EO serving a called party. All line numbers under a particular NPA-NXX are served from the same end office and are generally in the same local area. The LSP appearing in the Local Exchange Routing Guide (LERG) identifies the default LSP and EO associated with a particular NPA-NXX. The default routing for a call to a dialed number is to the default LSP for the dialed number. A Ported Number is an end-customer's number that is no longer served by the default LSP defined in the Local Exchange Routing Guide (LERG). An NPA-NXX is defined as being portable if it has one or more ported numbers. That is, an NPA-NXX is considered to be portable when any subscriber previously served from an EO associated with the NPA-NXX moves to another LSP and keeps the same 10-digit NPA-NXX-XXXX.
2.2 Long-Term Number Portability
For the long-term, the Location Routing Number (LRN) approach, which uses AIN (Advanced Intelligent Network) concepts, is being developed for addressing the service provider portability and the restricted geographic portability forms of NP. A Location Routing Number is an identifier for an LSP EO. The LRN of an LSP EO is defined as a 10-digit number having the format NPA-NXX-XXXX, such that the fist six digits is an NPA-NXX assigned to an LSP EO. A 6-digit translation on the NPA-NXX of the LRN results in routing calls to the LSP EO.
An NP Service Control Point (SCP) database is a Number Portability database storing number portability data, such as the LRNs for the EOs serving the ported numbers. In the long-tem, for interswitch calls, switches will query an NP database with a destination member (DN) for obtaining NP and routing information for that number. A destination number can be the dialed number, or the number derived after service processing, such as, the translated number after "800" service process has been applied In the case of ported DNs, queries to an NP SCP database return the LRN of the recipient switch to which a number has been ported. In the case of non-ported DNs, the queries cause the NP database to return the actual DN and not an LRN. Thus, a call to a non-ported DN is routed based on the dialed number while a call to a ported DN is routed based on the LRN.
The proper routing takes place because when a querying switch receives an LRN, the LRN is used to populate the Called Party Number (CdPN) parameter of the SS7 (Signaling System 7) ISUP (ISDN User Part) IAM (Initial Address Message). The CdPN parameter is an existing SS7 ISUP IAM parameter presently carrying the destination or directory number. Switches translate the number contained in the CdPN parameter for determining bow to route a call. In the long-term LRN approach, the dialed number originally used in the query of the database is "preserved" by string it in the Generic Address Parameter (GAP) of the IAM. The GAP is an existing optional SS7 ISUP IAM parameter. Multiple GAPs can be defined for a call. Additionally, the M bit of the Forward Call Indicator (FCI) parameter of the IAM is used for indicating whether an NP query has been performed so that more than one NP query is prevented from being launched during the setup of an interswitch call to a portable NPA-NXX.
FIG. 1 shows a call flow mechanism for the long-term LRN approach for an illustrative intraLATA call. In FIG. 1, Station A is served by a Local Service Provider 1 (LSP1). At 11, Station A dials the number of Station B. Station B has a number that has been ported from LSP2 to LSP3, as indicated by the broken dashed line between LSP2 and Station B. At 12, the originating switch launches a query to an NP database based on the dialed number being an interswitch intraLATA call and having a portable NPA-NXX. Since the dialed number has been ported, the NP database returns an LRN for the dialed number at 13. The returned LRN is used to populate the Called Party Number (CdPN) parameter of the IAM. The dialed number is used to populate the Generic Address Parameter (GAP) of the IAM. To avoid further NP database queries, the M bit of the FCI NP Query Indicator is set to "1". At 14, the signaling switch routes the call to a tandem switch based on the first six digits of the LRN. The tandem switch routes the call to LSP3 based on the first six digits of the LRN at 15. The LSP3 EO serving the ported subscriber recognizes its own LRN in the CdPN, obtains the dialed number from the GAP of the IAM, and completes the call to station B at 16.
2.3 Near-Term Number Portability
A common near-term, interim NP scheme is called Remote Call Forwarding (RCF). Remote Call Forwarding is a standard Local Exchange Carrier service that allows every call to a DN at an EO to be redirected to another DN in the same or a different EO. The RCF approach is only an interim NP scheme because it inefficiently routes every call through the subscriber's original serving EO, and because there are some adverse feature interactions between RCF and end-user Custom Calling Services, such as Caller ID and Automatic Call Return. Also, the conventional RCF approach requires use of two DNs per subscriber, that is, a "real" directory number that can be found in a telephone book, and a "shadow" directory number used for both rerouting the call to the switch serving the ported customer and completing the call to the proper customer. The base EO, as determined by the dialed DN, processes the real DN, determines that the call should receive RCF redirection, and reroutes the call based on the shadow DN. The terminating EO, now serving the ported customer and having an NPA-NXX matching the NPA-NXX of the shadow number, completes the call to the customer's station via normal switch translation.
FIG. 2 shows a call flow mechanism for the RCF approach for an illustrative intraLATA call. Station C, with a DN of 908-484-4811 and being originally served by LSP1, is ported to LSP2, as indicated by the dashed line. LSP2 assigns to station C a line shadow number of, for example, 908-949-1111. As part of the porting process, the RCF feature for Station C's DN is activated on the LSP1 EO2 originally serving station C. The LSP2 EO treats the shadow number associated with Station C like any other assigned line number. Station A, served by LSP1, dials the DN for Station C at 21. At 22, LSP1 EO1 routes the call based on the NPA-NXX for LSP1 EO2, which originally served Station C. At 23, LSP1 EO2 redirects the call to the Terminating EO based on the shadow DN, that is, to LSP2 EO3.
The conventional RCF approach suffers from the problem of the so-called trombone effect. To illustrate this, consider the situation shown in FIG. 2 when a Station B on EO3 dials the number of Station C at 24. Based on the first 6 digits of the DN, the serving EO, that is, EO3, has no indication that Station C has been ported and uses standard routing. Consequently, the call is routed to LSP1 EO2, which originally served Station C, based on the DN's NPA-NXX at 25. EO2 redirects the call based on its shadow number back to the LSP2 EO3, which now serves Station C. The call is routed correctly, but inefficiently in the trombone effect. The trombone effect takes place when the called number is ported from a first LSP to a second LSP, and the calling number originates on a switch that also serves the ported customer. The call then tandems through the ported customer's former switch (associated with the first LSP) before being routed back to the second LSP.
To avoid the trombone effect, a switch must be able to recognize that a call to a ported number is actually an intraswitch call, not an interswitch call. One way for a switch to recognize that a call is an intraswitch call is for the switch to do a 10-digit translation.
What is needed is a way to handle interim NP calls without using duplicative shadow numbers while also avoiding the trombone effect.